FLOW, LEVEL & CONTROL FEATURE
The ultimate top 10 LEVEL SENSING TECHNOLOGIES
Greg Wainhouse from Bürkert UK selects his top 10 level sensing technologies, explaining why design is key
a compensation factor is used by the on- board electronics to derive the precise level within the vessel. In applications involving changing atmospheric gases, foaming liquids or objects in the path of the ultrasonic pulse, reduced reliability and repeatability may be experienced.
L
evel sensing is one of the most common functions for process control
but it also uses a wide range of different technologies to achieve the same goal. As such, it is important to select the correct design for each application to ensure a reliable and accurate system.
1. FLOAT The float is one of the most common forms of point level detection, and although simple in design, offers a reliable solution. The actual switching mechanism can either be internal and activated by the movement of the float as the level rises, or external where a magnet switches a reed contact as the position of the float is moved by the liquid level.
2. TUNING FORK This sensor is set to vibrate at its mechanical resonance frequency and, when it becomes submerged in either liquid or solid, the frequency changes. This change is detected and converted into a switching command by the on-board electronics.
3. ULTRASONIC The transducers in these devices emit ultrasonic pulses that are reflected off the surface of the liquid - the time taken is proportional to the distance between the sensor and the liquid. This time is also affected by the temperature of the atmosphere above the media the soundwave has to travel through, and
Selecting the most appropriate level sensor requires a number of variables to be considered
4. RADAR In principle, radar operates in a similar way to ultrasonic, but the pulses travel at the speed of light. Again, the reliability and repeatability can be affected - but this time by the dielectric constant of the fluid. However, radar can provide very accurate level data and also compensate for fixed structures within the vessel. While the initial cost of the sensor is relatively high, several manufacturers are making this technology more accessible.
5. GUIDED MICROWAVE Also known as guided radar, this overcomes many of the initial calibration issues associated with conventional radar sensing by using a cable or rod to guide the signal to the fluid surface and back to the transmitter. This design makes it a lot more reliable than through-air radar or ultrasonic because the guide provides a more focussed signal path. The sensors are limited to the length of cable or rod, but with solutions up to 75m there is a wide array of probes to suit many applications.
6. LASER This technology is a flexible solution for non-contact continuous level detection of bulk solids and opaque fluids. However, being based on light reflection, it is less well suited to clear liquids, dust and vapour contaminated atmospheres and applications affected by foam.
7. HYDROSTATICS Hydrostatic pressure is one of the most popular level sensing solutions on the market for liquids. Used in vessels open to atmosphere, they are unaffected by changes in media dielectric, atmospheric dust and vapours. As they are mounted
on the base or underside of the vessel and measure head-pressure via a ceramic or thin-film stainless steel element, the reliability of measurement technique for level will be greatly affected if the media density changes.
8. DIFFERENTIAL PRESSURE Conventional hydrostatic sensors are used when the storage vessel is open to atmospheric pressure. For situations where a pressurised tank is in use, it is necessary to measure the atmospheric pressure above the media using a second sensor. The two pressure signals are then evaluated to give the actual hydrostatic pressure of the liquid and the corresponding level. This method is subject to errors due to changes in density and temperature but these can be mitigated by compensating factors, such as additional sensors.
9. CAPACITANCE This approach can be used with solids, liquids and mixed materials which act as a capacitor. The sensor needs to be calibrated to the specific material to account for varying dielectric constants and differences in vessel design. More recent developments have produced a contact-free measurement system that has widened its application. The accuracy of a capacitance sensor depends on a homogeneous dielectric constant throughout the storage vessel. The reliability of these sensors can also be heavily influenced by media sticking to the sensitive probe.
10. LOAD CELL The use of strain gauges provides a non-contact measurement system that is built into the support structure of the storage vessel. This also means that the design and construction of the vessel must account for the use of load cells in order to deliver accurate level measurements. Additionally, changes to product density will greatly affect the reliability of the measurement in the vessel.
HELP IS AT HAND Selecting the most appropriate level sensor requires a number of variables to be considered, and different technologies available can make the selection process quite daunting. Fortunately, help is at hand from the experienced engineers at Bürkert who have considerable expertise in designing and installing process control systems for a wide variety of applications.
Bürkert T: 01285 648720
www.burkert.co.uk
INSTRUMENTATION | DECEMBER/JANUARY 2017 15
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